The story revolves around Train Heartnet, a.k.a Black Cat. Chained down as a professional assassin, Black Cat is expected to adhere strictly to the orders of Chronos Organization. Betrayal to the groups ideals is unforgivable and leads only to death. Train lived his life, killing professionally without a second thought until he meets a 'Sweeper', Minatsuki Saya.
Being completely free and independent Saya believes strongly in choice, whether in taking charge of ones own life or choosing to take others'. With this contradiction, Train begins to doubt his methods and slowly yearns to search for more than cold murder.
Black Cat portrays different meanings of justice, honour and life as Train escapes from the clutches of indomitable Chronos and obsessive ex-partner to a "mercenarious" life of a sweeper.
